1. Field of the Invention
The present invention relates to a resin-sealed semiconductor device and in particular to a high power semiconductor device of the resin-sealed type having a heat dissipation plate provided at a lower surface of a resin-sealing portion.
2. Description of the Prior Art
As is well known, in the case of the power semiconductor devices and the power integrated circuit devices (power IC's), a large amount of heat is generated because of the large power consumption during use thereof. Since the semiconductor devices are likely to vary electric characteristic under the influence of heat, it is necessary to dissipate heat as rapidly as possible.
Heretofore, there has been known a power IC of the structure shown in FIG. 1 which is so constructed as to dissipate rapidly the heat generated in semiconductor elements. Referring to the figure, the power IC denoted generally by reference numeral 1 comprises a planar heat dissipating plate 2 of copper on which semiconductor elements are mounted. A plurality of leads 3 arrayed substantially in parallel to one another are provided above the plate 2 and connected to the associated semiconductor elements at the inner ends by means of wires or the like. The upper portion of the heat dissipating plate 2 is coated with a resin material. In other words, the inner end portions of the leads 3, the connecting wires and the semiconductor elements are covered by a resin-sealing portion 4. However, at least the lower surface 5 of the heat dissipating plate 2 is exposed as shown in FIG. 1. It is to be noted that the power IC 1 is shown as upset in FIG. 1. so that the lower surface 5 of the heat dissipating plate 5 may be observed clearly. The heat dissipating plate 2 as well as the resin-sealing portion 4 are formed with notches 6 adapted to receive set screws at both ends thereof.
The hitherto known power IC of the structure described above suffers from drawbacks that cracks are frequently produced at a center portion of the upper surface of the resin-sealing portion and in an extreme case even the semiconductor elements in the resin-sealing portion are damaged or cracked, when the power IC is mounted on a mounting plate by means of the clamping screws.
From examinations to clear up the causes of such undesirable phenomena, a following fact has been found. Namely, when the resin-sealing portion is formed during the fabrication process of the power IC, the sealing is carried out at a high temperature, which is subsequently decreased to room temperature. At that time, a warp or bending will occur in the resin layer due to difference in thermal contraction between the heat dissipating plate and the resin layer and additionally due to the setting contraction of the resin when the temperature is lowered. In more particular, the thermal expansion coefficient of the heat dissipating plate of copper is 17 .times. 10.sup.-6 /.degree. C which is very much smaller than that of the resin. The latter has usually a very large thermal expansion coefficient on the order of 30 .times. 10.sup.-6 /.degree. C, for example. For this reason, the resin portion is subjected to a great contraction after the sealing, which gives rise to the warp or bending of the resin layer. Assuming by way of example that the resin layer has width of 10 mm, length of 30 mm and thickness of about 5 mm, while the heat dissipating plate is formed of a copper plate of 1 to 1.5 mm thick with the width and the length being substantially equal to those of the sealing portion or slightly smaller than the latter, the warp a (FIG. 2) produced in the resin layer will amount to about 0.2 mm as measured at the end portions of the resin layer.
Such being the circumstances, when the power IC having the warped resin-sealing portion is to be secured onto the mounting plate 9 at both ends by means of the clamping screws 7 and nuts 8, a great tensil stress will be produced in the resin-sealing portion adjacent to the upper surface thereof. When the tensile stress exceeds the tensile strength strain of the resin on the order of 4000 .times. 10.sup.-6, cracks 10 will be produced, resulting in a reduced moisture resistance of the power IC. Besides, when the warp is of a greater magnitude, then the semiconductor elements embedded in the resin-sealing portion will be also damaged, resulting in a deterioration in the electric characteristics thereof and eventually non-usability.
In connection with the above discussion, it is conceivable that the sealing portion is bent along the edge portions thereof when the sealed product is taken out of a defective sealing mold. Such a bend will of course provide a cause for cracks in the upper surface of the sealing portion, etc.